The power to mass ratio (specific power) is an important metric for electric motors, especially for those used to power electric and hybrid vehicles. As specific power is increased, motor mass can be reduced while maintaining a given level of performance. This provides both direct and indirect economic benefits.
Power is equal to the product of shaft speed and torque, and power (and hence specific power) may be increased by increasing both speed and torque. For speed, the ultimate limit is where centrifugal stress causes the rotor to mechanically fail. This limit is different for different motor types. For induction motors, the rotor structure is relatively strong and hence relatively high speeds can be safely tolerated. Rotor surface speeds of 150 m/sec may be possible for some designs.
The ultimate torque limit is also different for different motor types. The torque limit may be proportionate to the square of the gap magnetic flux density. Other factors also affect the torque. In the case of induction motors, leakage inductance is a factor; peak torque may vary inversely with this parameter. Hence, for induction motors, higher specific torque is achieved by reducing the leakage inductance.
Iron losses are produced which are approximately proportionate to the square of the electrical frequency. Since speed (rpm) is proportionate to the electrical frequency, it follows that iron losses vary approximately with the square of the shaft speed. Likewise, conductor losses are approximately proportionate with current. In turn, torque is approximately proportionate to current (under conditions of maximum magnetic flux density). Hence, conductor losses vary approximately with the square of the torque. It therefore follows that if speed and torque are maintained proportionate to each other, efficiency remains constant. Thus, for example, if both speed and torque are doubled, both the loss and through-power increase by a factor of four, while efficiency remains approximately constant. Heat transfer, however, must also be increased by a factor of four. Thus, there is a need for a system for providing effective cooling in an electric motor.